Magnetic memory devices use magnetic storage elements to store data rather than electric charge or current flows as is the case with electronic memory devices.
Magnetoresistance is the property of a material to change its electrical resistance under the influence of an external magnetic field. The Giant magnetoresistance (GMR) is a type of magnetoresistance that manifests as a significant change in electrical resistance in the presence of an applied magnetic field. GMR occurs in thin film structures composed of alternating ferromagnetic and non-magnetic metal layers. The tunnel magnetoresistance effect (TMR) occurs when two ferromagnets are separated by a thin (about 1 nm) insulator, in which case the resistance to a tunneling current changes with the relative orientation of the two magnetic layers. The resistance is normally higher in the anti-parallel case.
Magnetic memory devices may be fabricated in which a magnetic (hereinafter also “magnetoresistive”) element defined by a thin film structure comprising ferromagnetic materials selected to have magnetoresistance (either GMR or TMR) is used to store data.
One such magnetic element is known as the Magnetic Tunnel Junction (MTJ). Fundamentally, the MTJ is a structure with a magnetic layer whose magnetization can be manipulated to store data. The MTJ may be realized as a stack comprising a pinned layer with fixed magnetism, a tunnel oxide layer and a free layer. Typically, a memory device will have millions of MTJs arranged in the grid. For illustrative purposes, FIG. 1 of the drawings shows such a grid in the form of an exemplary 3×3 grid/array 100. In the array 100, the MTJs are represented schematically by resistors 102. For writing, two orthogonal currents carried by a word line (WL) and the bit line (BL) manipulate magnetic fields in the free layer. The word lines in the grid 100 are indicated as WL0 to WL2, whereas the bit lines are indicated as BL0 to BL2. When the magnetic orientation of the free layer coincides with that of the pinned layer, tunnel resistance of the MTJ becomes low. When the direction is opposite, the resistance becomes high. Thus, a memory high or low state may be defined as high or low resistance of the MTJ.
For reading purposes, each MTJ 102 in the grid 100 is connected to a transistor 104 which in turn is connected to a digit line (DL). In FIG. 1, the digit lines are indicated as DL0 to DL2.